The present invention relates to a polishing apparatus for fabricating a wiring substrate and a fabrication method of a wiring substrate using such apparatus, particularly to a polishing apparatus and a polishing method for fabricating metal wirings for a semiconductor device by using a polishing operation.
In recent years, importance is given to planarizing of a surface of a wiring substrate for a large scale semiconductor integrated circuit (hereinafter, described as LSI). A method of Chemical Mechanical Polishing (CMP, hereinafter, described as polishing so far as the description is not specified otherwise) is regarded as one of the representative technologies.
Further, as a method of fabricating copper or a copper-based alloy (Here, a copper-based alloy means an alloy in which the weight percentage of Cu in the materials which compose the alloy is larger than the weight percentage of any other material in the alloy; hereinafter, described as copper alloy.) for interconnect metal lines, a method referred to as the damascene method attracts attention and is disclosed in, for example, JP-A-2-278822. Here, a single damascene method is described in U.S. Pat. No. 6,004,880, the contents of which are incorporated herein by reference in their entirety. And a dual damascene method is described in U.S. Pat. No. 6,004,188, the contents of which are incorporated herein by reference in their entirety. Among the methods, in respect of polishing a copper alloy, a detailed description has been given in JP-A-8-83780.
Methods of polishing are roughly classified into methods of using abrasive powders and methods of substantially not using abrasive powders. The former is mainly used for planarizing a surface of a predetermined substrate or removing projected portions of thin films. The latter has mainly been used for removing damaged or degraded portions of a surface after polishing using abrasive powders as described in, for example, JP-A-9-306881. Further, conventionally, according to the polishing operation of not using abrasive powders, generally polishing speed is extremely low and therefore, although there has been a technology of polishing without using abrasive grains with an object of removing a predetermined layer after a polishing using abrasive powders and thereafter removing mainly a damaged layer of the surface, it has been regarded as practically difficult to remove the predetermined layer by the polishing of not using abrasive grains. For example, in respect of polishing of aluminum, there has been proposed a method of using an alkaline polishing solution without including abrasive powders in Japanese Laid Open No.2-580939. However, according to the method, the polishing speed is as low as 30 nm/min at maximum. Further, in a practical point of view, polishing with high accuracy is difficult unless the ratio of polishing rate to etching rate is larger than 20 hereinafter, described as selectivity ratio), but the selectivity ratio of the proposed method is as small as 3 or smaller. Further, there has been carried out a trial in which a copper alloy is polished by an abrasive free solution. A description has been given to an example of using nitric acid or a mixture solution of nitric acid and hydrogen peroxide which does not include abrasive in Proceedings of Abrasive Fabrication Society 1997, volume 41, pp. 231-233. However, nitric acid is a chemical used in etching copper and according to the solution, the etching rate is as large as the polishing speed and therefore, although an effect of smoothing a polished face can be expected, in using it in a process of so-to-speak damascene wiring the process is devoid of consideration in respect of the etching rate and is not suitable for practical use.
Further, there is disclosed a method of polishing a silicon wafer or a glass substrate by a method of not including abrasive powder in a polishing solution by using a polishing pad (described as grindstone) including fixed abrasive powders of silica, cerium oxide or the like in place of a polishing pad made of polymer resin in JP-A-10-125880 or JP-A-8-64562. Further, Proceedings of SEMI Technology Symposium, 1998 edition p. 5-72 to p. 5-78, describes also a method of polishing a copper alloy by using a similar grindstone. However, the specific content of the grindstone or the polishing solution used is not disclosed and the grindstone and the polishing solution are supplied together from the supplier of the consumables.
Paying attention to an aspect of polishing characteristics, slurries are roughly classified into alkaline slurries (described as mechanical effect slurry) mainly realizing the polishing characteristics by mechanical effects and polishing liquids (solutions) enhancing a role of chemical reaction. The former is mainly used in polishing silicon oxide or silicon. In contrast thereto, a chemical effect suspension is mainly used in selective polishing. According to the chemical effect suspension, normally, in addition to abrasive powder and a dispersant, a chemical effect suspension including acids or salts of these is mixed with an oxidant immediately before the use to thereby constitute a slurry and the slurry is used in polishing by injecting it to a platen for polishing. As the oxidant, ferric nitrate, hydrogen peroxide, potassium iodate or aqueous solutions of these are known. Hereinafter, in respect of the chemical effect slurry, a liquid before mixing with an oxidant is referred to as a suspension and a liquid after having mixed with an oxidant is referred to as a slurry as a discrimination therebetween when the discrimination becomes necessary.
However, in the case of a chemical effect suspension, abrasive powders are liable to aggregate in the suspension and it is difficult to provide uniformly dispersed suspension. Further, even in the case of a suspension which has once dispersed, the suspension is liable to become a nonuniform suspension or slurry by the aggregation or settling of the powders. When a slurry in a nonuniformly dispersed state is used, polishing scratches are made on the surface of a metal film or insulating layer and results in a failure, and therefore, technologies for preventing these, for example, technologies for providing a uniformly dispersed suspension by, for example, a kind and a concentration of a dispersant or by stirring a solvent including abrasive powder and chemical components for a predetermined period of time while maintaining at a predetermined temperature, become an important problem in the technical aspect for polishing suspensions.
Further, a chemical effect suspension is produced by mixing abrasive powder and a dispersant thereof, further, a plurality of kinds of acid or complex salt, or a protective layer forming agent and so on under strict composition control and therefore, it is extremely difficult for a user to control the chemical components. Such a chemical effect slurry including abrasive powder requires complicated and delicate production steps and a long period of time seems to be required in the production.
As mentioned above, in the case of the chemical effect suspension, the composition is complicated and therefore, in using the chemical effect suspension continuously in fabricating interconnect substrates, a chemical effect suspension is transported from a production site to a site of using the chemical effect suspension, contained in a predetermined reception apparatus and thereafter, mixed with an oxidant at a predetermined rate and supplied and used in a polishing apparatus as a chemical effect slurry. The reception apparatus mentioned here indicates a suspension or slurry supply unit for an individual polishing apparatus, a concentrated reception apparatus for supplying the suspension or slurry to a plurality of polishing apparatuses or the like. In this way, according to a polishing method using a chemical effect suspension, chemical components of a slurry are complicated and the composition control is also difficult and therefore, there poses a problem in which in addition to the cost of producing a suspension per se, a waste solution treatment of the polishing liquid requires high cost.
The inventors have found that polishing of metal can be carried out at a practical high speed even with a combination of a liquid (e.g., slurry) which does not include substantially abrasive powder in the suspension or slurry and a polishing pad made of polymer resin which does not substantially include abrasive powder. This abrasive-grain-free chemical mechanical polishing is described in U.S. patent application Ser. No. 09/182,438, the contents of which are incorporated herein by reference in their entirety. In this case, an abrasive free slurry signifies a slurry having a concentration of abrasive powder of less than one weight percent of combined weight of liquid and abrasive in respect of the slurry. When a particularly preferable composition is used, practical polishing can be carried out even with a concentration of abrasive powder equal to or smaller than 0.1 weight percent (having less than 1 weight percent concentration of abrasives powder, this is also referred to herein as abrasive free suspension or slurry).
By abrasive free slurry according to the present application, we mean a slurry having a concentration of abrasive powder of less than 1 weight percent of combined liquid and powder in respect of the slurry, which would include a liquid containing no abrasive powder (e.g., a polishing liquid such as a solution, with no abrasive powder). Thus, while throughout this application a suspension or slurry is referred to, within the contemplation thereof by the inventors is a polishing liquid in general, e.g., a polishing solution, containing no, or substantially no, particulate matter including abrasive powder.
(1) According to the present invention, in respect of the above-described abrasive free slurry, the abrasive free suspension per se need not be produced at a production site of the slurry, but materials constituting the abrasive free suspension or solutions of the materials for making the suspension are prepared and transported to a site where the suspension is used; successively, at the site of use, solutions of the materials are produced and the solutions are mixed with each other and diluted and mixed with an oxidant to produce and use as the predetermined abrasive free slurry.
By installing a production equipment of an abrasive free slurry at a site substantially the same place as a polishing apparatus in this way, that is, by making an abrasive free slurry at a portion of the polishing apparatus or at a vicinity of the polishing apparatus or in the same plant (factory), there is achieved an effect of reducing cost of transportation of the suspension from a production site to a site of use or cost of vessels. In case that the production equipment of an abrasive free slurry is installed in the vicinity of the polishing apparatus, the distance between them is desirably within 1 km substantially, preferably within 400 m. In other words, a polishing apparatus of the invention has a polishing unit, a mixture unit, which mixes polishing materials so as to prepare a liquid, supplied to the polishing unit, the liquid including abrasive powder concentration less than 1 weight percent of combined weight of liquid and abrasive powder, and piping directly connecting the polishing unit to the mixture unit. This piping directly connecting the polishing unit to the mixture unit includes the case that there is some object like a filter between the polishing unit and the mixture unit. Or a polishing apparatus has a polishing unit, and a mixture unit, which mixes polishing materials so as to prepare a liquid, supplied to the polishing unit, the liquid including abrasive powder concentration less than 1 weight percent of combined weight of liquid and abrasive powder, the mixture unit having a supplying structure which provides continuous flow communication between the polishing unit and the mixture unit.
Further, only raw materials for polishing are transported from a production site of raw materials for a suspension to a site of use and accordingly, the volume to be transported is significantly reduced. For example, chemical components included in an abrasive free suspension are several percent of the suspension at most and accordingly, compared with a case in which a completed abrasive free suspension is transported, the transportation volume can be reduced to about ten percent. Further, when polishing component materials are prepared by being divided into a single one of components for a suspension in the completed state or a solution of a single material having a concentration 10 times or more as much as that in use or a solution comprising a mixture of a plurality of kinds of materials for facilitating preservation or transportation and transported to a site of use, the difficulty in inspection at a production site is reduced and accordingly, cost of the abrasive free suspension or slurry, and therefore, fabrication cost of a wiring substrate, can be significantly reduced.
For example, an abrasive free suspension before being mixed with an oxidant contains polishing materials of a plurality of kinds of organic acids or salts of these, a protective layer forming agent and the like. Total concentration of these are less than 10 weight percent of the suspension at most. Accordingly, when a raw chemical for an abrasive free suspension is transported to a site of use individually or in the form of a partially mixed condensed solution by using the method of the present invention, there can be constituted a solution having a concentration ten times or more as high as that in use. Although a concentration constituting a limit differs depending on a polishing material, specifically, there can be constituted a solution having a dissolution limit in consideration of environmental temperature in transportation, for example, a concentration equal to or higher than 20 weight percent. In contrast thereto, benzotriazole (described as BTA) which is known as a representative protective layer forming agent, a concentration thereof added to a polishing solution is equal to or less than 0.5 weight percent at most and it is dissolved in water by about 2 weight percent at normal temperature. The effect is low since it can be condensed only by several times. With regard to such a material, a method of making a solution by transporting it in the form of a solid to a site of use is more advantageous. Although the transportation cost can significantly be reduced in the form of a solid state, handling at a site of use is more facilitated in the case of a solution state. Which material a user receives in which state may be determined in consideration of a fabrication apparatus according to the invention or an environment where the fabrication apparatus is installed.
Further, generally, in a chemical effect suspension, there poses a problem in which reaction among components is progressed and properties of the polishing solution are liable to change by aging. However, by using the method of the present invention, a slurry can be used in a short period of time after it has been prepared, and, accordingly, there is no need of considering an aging change or adding a stabilizer for extending life of the solution.
According to an example, which has frequently been used in recent years in polishing tungsten, a suspension including silica abrasive powder and an aqueous solution of hydrogen peroxide are mixed and used in polishing. According to the slurry, ferric nitrate used conventionally may not be used as an oxidant, and therefore, it has been expected that adverse influence thereof on characteristics of devices may not be considered. However, it has been found that ferric nitrate of about several tens to several hundreds ppm has to be added to the suspension. Ferric nitrate seems to have also an effect of a dispersant of abrasive powder. Therefore, it has been found that the adverse influence on device characteristics caused by contamination of iron cannot be disregarded. Further, according to a slurry including fine cerium oxide abrasive powder (having an average particle size of 0.3 micrometer or smaller) which is widely used in polishing an insulating film, although the abrasive agent immediately after dispersion shows excellent characteristics, after preserving it for about two months after dispersion aggregation occurs therein and a number of scratches are generated. Although various kinds of surfactants are added in order to prevent this, the content has not been disclosed. According to a surfactant of sulphonic acid or polyacrylic acid or the like, which is widely used, the polishing speed is lowered. In this way, in order to stably disperse abrasive powder, control of particle size or addition of a dispersant is needed although it is not preferable in view of polishing performance per se. This is to guarantee the life of the suspension for about three months or more. In contrast thereto, an abrasive free suspension does not include abrasive powder and therefore, only components necessary for polishing may be included.
The abrasive free slurry includes polishing abrasive powder of less than one weight percent, an oxidizing substance, a substance for making an oxide water soluble and a corrosion preventing substance. This corrosion preventing substance is described in Japanese Laid Open Number 8-64594, the contents of which are incorporated herein by reference in their entirety. Actually, a metal film on an insulating film is polished by using a slurry including abrasive powder of less than one weight percent, an oxidizing substance, a substance for making an oxide water soluble and a corrosion preventing substance in which pH and oxidation-reduction potential fall in a corrosion area of the metal film. When the slurry is used, not only a sufficiently practical polishing speed can be provided by a combination with a conventional polishing pad but also there is achieved an advantage in which scratches are difficult to be made on a polished face. Particularly, the slurry is preferable in forming metal wirings of an LSI using the damascene method. Further, abrasive powder is not included, and, therefore, reduction in cost of a suspension and simplification of handling are also expected.
An abrasive free suspension is prepared by dissolving and mixing various kinds of polishing materials in a solvent, and an abrasive free slurry is prepared by mixing the suspension with an oxidant under a diluted state as necessary. Further, the order of mixing is not limited thereto. According to the abrasive free slurry, all of the included components are dissolved and it has been found that production apparatus for supply thereof can be provided comparatively in small size and inexpensively.
In this case, as an oxidizing substance of the abrasive free slurry, hydrogen peroxide solution is most preferable since it does not include metal components and is not a strong acid. Although ferric nitrate or potassium periodate includes a metal component, it achieves an effect of increasing the polishing speed since the oxidizing power is strong.
Further, as a substance for making an oxide water soluble, acid is pointed out, and operation thereof for making an oxide water soluble as metal ions (for example, Cu2+ ions) is utilized. There are inorganic acid, organic acid and salts of these. Specifically, although benzoic acid, oxalic acid, malonic acid, succinic acid, adipic acid, pimelic acid, maleic acid, phthalic acid, malic acid, tartaric acid, citric acid and salts of these or mixtures of these are particularly effective, the substance is not limited thereto.
These acids or complex compounds may constitute a solution of a polishing material having a predetermined concentration in a mixture vessel regardless of whether they are in a solid state of in a solution state. These materials are dissolved in water comparatively easily.
Next, as a corrosion preventing substance, that is, the protective layer forming agent, in respect of a copper alloy, BTA is known most widely and shows the strongest effect. Otherwise, one or a plurality of substances selected from the group consisting of polymers including monomers having carboxylic acids of BTA derivatives, polyacrylic acid and salts of these are effective.
(2) Next, an explanation will be given of a specific polishing step. First, one or a plurality of solutions or solids of polishing materials are dissolved in a solvent individually or in a mixed state to thereby prepare an abrasive free suspension and the abrasive free suspension is diluted by using a solvent as necessary. Or, when there are a plurality of kinds of solutions of polishing materials, these are mixed to prepare an abrasive free suspension, the abrasive free suspension is diluted as necessary and the abrasive free suspension and an oxidant are mixed to thereby prepare an abrasive free slurry. However, an oxidant may not be constituted only by the oxidant but may include at least portions of components of polishing materials (a mixture solution of an oxidant or a portion of a component of a polishing solution whose major component is an oxidant, which differs from a composition finally used in polishing, is summarizingly described as an oxidant). The abrasive free slurry prepared in this way is supplied to a polishing apparatus.
Successively or separately therefrom, a wiring substrate formed with an insulating film having a predetermined thickness on its surface is prepared, grooves for wirings are formed in the insulating film and one layer of a metal layer for wirings or metal layers for wirings substantially including an upper metal layer and a lower metal layer are formed and embedded into the grooves. xe2x80x9cSubstantiallyxe2x80x9d described here signifies that the metal layers are classified into metal layers having different polishing characteristics and an upper or a lower metal layer does not need to be constituted by a single layer, respectively, so far as the polishing characteristic is similar. Further, when the wiring metal layers are divided into more layers in view of polishing characteristics, the method of the present invention in respect of a lower metal layer may be used as an application. An explanation will be given of a case of comprising upper and lower metal layers as follows.
Further, when the wiring metal material is mainly aluminum, the ratio of the polishing speed to the etching rate which determines the polishing accuracy is small and is not sufficiently practical; however, low cost formation can be achieved by using the method of the present invention. Further, when the wiring metal material is tungsten, the polishing operation can similarly be carried out by optimizing a composition of an organic acid or the like according to the present invention.
Successively, the wiring substrate is pressed to a polishing platen while injecting the abrasive free slurry and at least a portion of an upper metal layer is removed. When a lower metal layer is difficult to polish by the abrasive free slurry, another kind of an abrasive free slurry may further be supplied. Particularly when the lower metal layer is made of a material which is difficult to polish such as tantalum (described as Ta), the wiring substrate may be moved to another platen and polishing may be carried out further by using a slurry including conventional abrasive powder. When only the abrasive free slurries are used, a plurality of abrasive free slurries may be used on the same platen or other platens may be used respectively. Further, the lower metal layer may be removed not by polishing but by etching.
Further, according to the present invention, there also is achieved an advantage in which polishing conditions can continuously be changed during polishing. In the case of the abrasive free slurry, the polishing speed of a metal layer can be increased by a method of increasing a concentration of an acid or a complex salt or reducing a concentration of a protective layer forming agent. However, there is a tendency of also increasing the etching rate, and, accordingly, when excessive polishing is carried out, dishing is liable to increase considerably at the end of metal polishing. Accordingly, at an initial stage of polishing, the polishing is carried out until the surface of the metal layer becomes substantially flat under conditions of a combination of high polishing speed and high etching rate, successively, the polishing operation is carried out by changing the conditions to a low etching rate, by which the polishing operation can be carried out in a short period of time and further, an increase in dishing can be restrained in excessive polishing.
Further, high speed polishing can be carried out in respect of various kinds of materials by applying the abrasive free slurry also to polishing using a grindstone.
Further, from another aspect, within 24 hours from preparing an abrasive free slurry by mixing, the abrasive free slurry is used to polish a film. Desirably the abrasive free slurry is used to polish a film within 4 hours from preparing the abrasive free slurry, because the abrasive free slurry contains the oxidizing substance so that the chemical reaction progresses by mixture with an oxidizing substance gradually.
(3) Successively, an explanation will be given of a site of installing a supply system of an abrasive free suspension. When various kinds of polishing materials for preparing an abrasive free suspension are powders, a portion of preparing the supply system for the solutions of the polishing materials are installed outside of a clean room for fabricating wiring substrates and when a successive system, handling the solutions of the polishing materials, is installed in the clean room, an area occupied in the clean room which needs expense in administration can be reduced. The reason why, a portion other than the portion of preparing the solutions from the powders is installed in the clean room is that in view of polishing step operators, the operational performance is promoted by increasing the ratio of the portion of the system installed in the clean room. Conversely, in view of reducing the cost of administration of the clean room, for example, all of portions of the supply system for the abrasive free suspension should be installed outside of the clean room and only the abrasive free suspension is introduced into the clean room, filtered and thereafter mixed with an oxidant to use for a polishing apparatus by which the occupied area can substantially be minimized.
Further, as a mode intermediary therebetween, it is also possible that the portions of the system for preparing the solutions of the polishing materials are installed in sections in the clean room having a lower grade than those of other areas and having a lower administration cost, or installed at sections provided with a measure of not effecting adverse influence on other portions by installing an enclosure or a local exhausting facility. In this case, intermediary advantage and cost can be realized in view of administration cost and operational performance of the clean room.
In this case, the abrasive free suspension may be mixed with an oxidant after having been introduced into various polishing apparatus or may be introduced into various polishing apparatus after having been mixed with an oxidant.
(4) Successively, an explanation will be given on the supply system of suspension for realizing the present invention. First, when a polishing material is a solid, it is suitable to use a mixing vessel for preparing a solution by adding a predetermined amount of water to the polishing material. A stirring function or another function may be provided for expediting dissolution; for example, when water temperature is elevated, the dissolving may be enhanced. A method of accelerating dissolution of a material which is difficult to dissolve is not limited thereto but pertinent function may be added to the mixing vessel. When all of included components are put into the same mixing vessel and dissolved in a solvent in one operation by predetermined rates of concentrations, the abrasive free suspension can immediately be provided. Further, when a plurality of kinds of solutions are prepared, the abrasive free suspension can be constituted by mixing these by predetermined rates. Further, when polishing materials are supplied in the form of liquid, the individual mixing vessel may only be provided with a function of constituting a predetermined concentration by mixing with water. Naturally, stirring function may be provided as necessary. Further, as stirring function, there can be used a method of rotating a rotor such as a propeller in a liquid or a method of circulating the polishing solution by using a pump. The above-described abrasive free suspension may be prepared at a concentration which can be used as it is or a concentration at an intermediary stage which is higher than a concentration in actual use in consideration of easiness of handling such as size of the vessel. Further, as a method of constituting the abrasive free suspension by mixing solutions of polishing materials, there can be used a method of taking out continuously the solutions of the individual polishing materials by predetermined flow amounts per unit time and mixing them to each other (continuous mixing method) or a method of taking out respective predetermined amounts of the solutions of the individual polishing materials and mixing them (batch type mixing method) When a pump by which a constant volume is transferred per cycle(referred as, constant volume pump) is used, a pulsating flow is produced and therefore, the batch type mixing is suitable, however, a pump weakening a degree of a pulsating flow is on sale and when such a pump is used, the continuous mixing method may be used. The prepared abrasive free suspension is transferred to a second mixing unit for mixing with an oxidant. A method of mixing thereof may be the continuous mixing method or the batch type mixing method. The prepared abrasive free slurry is preserved in a buffer vessel as necessary.
As another example of a method of preparing the abrasive free suspension, there may be used, for example, a method of preparing a solution of a polishing material which is difficult to dissolve in water and successively adding other polishing materials to the solution. In mixing the abrasive free suspension with an oxidant, it is preferable to dilute and prepare the abrasive free suspension previously to a concentration of use. Because when the abrasive free suspension is at a high concentration, reaction with an oxidant may be expedited and life of the slurry may be deteriorated. Naturally, when the degree of deterioration is retarded to a nonproblematic degree, the abrasive free suspension may be mixed with the oxidant before dilution. Further, in the case of a solution of a pertinent polishing material which is slow to react with an oxidant, the solution may be mixed with the oxidant before mixing all of components of the abrasive free suspension.
The abrasive free slurry prepared in this way may be supplied directly to one or a plurality of polishing apparatus by pipings or may be transferred temporarily to another vessel and transported and supplied to the polishing apparatus which are not connected directly thereto.
When a pump for taking out the above-described solutions of the polishing materials or the abrasive free suspension from the mixing vessel is a tube pump or a constant volume pump, the flow rate may be oscillated as in a pulsating flow and stable mixing may not be carried out or predetermined rates of components may be deviated only by coupling pipings. In such a case, when a buffer tank for mixing is installed and stirring function is provided as necessary, it is preferable in stabilizing the composition. In order to promote the accuracy of controlling the composition of the abrasive free suspension, it is preferable to make concentrations of solutions of polishing materials near to low concentrations such that the abrasive free suspension having a predetermined concentration is prepared only by mixing them. However, when the accuracy of controlling a constant volume pump or the like for taking out solutions of polishing materials is sufficiently high, concentrations of the polishing materials may be made high and constant amounts thereof may be taken out by pumps and diluted. This method is suitable for preparing a large amount of a suspension. It is preferable that a control deviation of a pump is plus or minus 10 percent or less in respect of a predetermined flow rate (average value when a pulsating flow is constituted). Further, when individual polishing apparatus are attached with a diluting portion for the abrasive free suspension by water, the abrasive free suspension at an intermediary high concentration may be prepared in a supply system. Further, when individual polishing apparatus are attached with a function of adding an oxidant to the abrasive free suspension, only the abrasive free suspension may be prepared by a supply system and supplied to the polishing apparatus and the abrasive free slurry may be constituted in the polishing apparatus.
Further, it is eftective to remove foreign matters in the abrasive free suspension or the abrasive free slurry by filtration as necessary at a final stage immediately before mixing with an oxidant on the outlet side of the mixing vessel, or a vessel of the suspension. According to the conventional slurry, abrasive powder is included and accordingly, a mesh size of a filter has been very coarse to a degree of 10 micrometers at least. In contrast thereto, according to the method of the present invention, abrasive powder is not originally included in the slurry and accordingly, it is easy to use a filter having a mesh size of 1 micrometer or less. Since such a fine mesh filter can be used, a number of foreign matters in the slurry is considerably reduced, which is effective in reducing polishing scratches. Further, a filter is generally made of polymer resin, and deterioration is liable to progress when it is exposed to an oxidant; accordingly, the filter may be placed at a stage before mixing with the oxidant. However, it is more effective for removing foreign matters to place a filter after mixing with an oxidant, and, accordingly, when a preference is given to removal of foreign matters, a frequency of changing the filter may be increased. Further, there is. resolved a problem in which foreign matters adhered to a vessel in transporting a suspension from a site of producing the suspension, are mixed into the suspension and mixed in transferring to a reception apparatus to thereby cause damage on wiring substrates in polishing.
Further, according to the present invention, a supply system connected to polishing apparatus may be of a plurality of routes. Polishing by using the abrasive free suspension can be carried out not only for a copper alloy constituting an upper metal layer but also for a lower metal layer as a barrier comprising tungsten or titanium nitride and the supply system according to the present invention can be used for polishing the lower metal layer. Conversely, a plurality of polishing apparatus may be connected to one route of a supply apparatus.
When an applied wiring substrate is a silicon wafer for a semiconductor integrated circuit, contamination by alkaline metal, alkaline earth metal or halide is not preferable, it is preferable to restrain amounts of these substantially to 10 ppm or less, and, accordingly, acid or its ammonium salt is preferable; however, contamination can be restrained actually to a nonproblematic level by cleaning technology using a liquid having slight reactivity with a copper alloy. However, cost of a cleaning step is slightly increased. When a wiring substrate is a glass substrate, such a problem may not occur.
In a total amount of a composition of a slurry according to the present invention, concentrations of organic acid and a protective layer forming agent such as BTA or other dissolving components are in a range of 0.001 through 5 percent in respect of water. Their concentrations are very low and accordingly, a large mixing vessel is required to provide a polishing solution having a predetermined amount and charging of materials and taking out of a dissolved suspension must be repeated frequently. In contrast thereto, when solutions of polishing materials having intermediary concentrations twice or more as much as concentrations of use, are prepared and respective predetermined amounts thereof are taken out by a tube pump or the like, mixed and diluted to thereby prepare the abrasive free suspension, downsizing of a mixing vessel can be carried out or, conversely, a frequency of steps of charging, dissolving and diluting polishing materials can be reduced.
Further, many of saturated solubilities of respective polishing materials reach 10 percent or higher except that the saturated solubility of a portion of a protective layer forming agent is several percent. In this case, a method of preparing a solution of a polishing material having a low solubility such as BTA separately from solutions of other polishing materials and mixing both to thereby constitute the abrasive free suspension, is effective. Further, the abrasive free suspension may be prepared by preparing solutions of polishing materials for respective components constituting the abrasive free suspension and mixing these.
Further, in place of a polishing pad made of polyurethane resin which is generally used in polishing, a grindstone can also be used. Cerium oxide in addition to alumina or silica can also be used for abrasive powder included in such a grindstone. When such a grindstone is used, planarity can significantly be promoted than in the case of using a resin pad. However, a grindstone is generally porous, absorbs components of a polishing solution and may effect adverse influence when polishing is repeated or when a polishing solution is changed, particularly in the case of using a chemical suspension. As a countermeasure, there is a method of increasing a supply amount of a chemical suspension or adjusting a composition of a chemical suspension which is supplied such that influence by absorbed components is preserved. In the former case, it is necessary that cost of the chemical suspension is low and in the latter case, it is necessary to flexibly adjust a composition rate of the chemical polishing solution. Although these are difficult to deal with according to a method of transporting and using a completed suspension as in the conventional case, according to the supply system and the fabrication method of the present invention these can be dealt with comparatively easily.